It is known from U.S. Pat. No. 2,861,900, issued Nov. 25, 1958, entitled "JET PLATING OF HIGH MELTING POINT MATERIALS", that particles can be heated to high temperatures by being entrained in the combusting mixture and in the jet flame with an appreciable temperature increase corresponding to the kinetic energy expended upon the impact of the high velocity particles upon the surface of the workpiece to be coated sufficient to ensure a firm mechanical bond with the surface of the workpiece.
In my U.S. Pat. No. 5,120,582, issued Jun. 9, 1992, entitled "MAXIMUM COMBUSTION ENERGY CONVERSION AIR FUEL INTERNAL BURNER" and in my U.S. Pat. No. 5,271,965 entitled "THERMAL SPRAY METHOD UTILIZING IN-TRANSIT POWDER PARTICLE TEMPERATURES BELOW THEIR MELTING POINT", unlike U.S. Pat. No. 2,861,900, the particles are fed into the jet stream downstream of the throat of an elongated expansion nozzle having a L/D ratio of least 3:1 to prevent clogging of the nozzle bore.
In U.S. Pat. No. 5,271,965 there is particular emphasis on impact fusion, i.e. the method of producing a coating by impacting high-velocity solid (plastic) particles against the surface in which the released impact energy raises the particles to their melting point. In that application, it is noted that "impact fusion" is best carried out by injection of the powder being sprayed into a supersonic jet stream of a static temperature less than that of the melting point of the powder being sprayed. For example, operating an oxy-fuel internal burner at a combustion pressure of 300 psig produces a 6,700 ft/sec jet with a static temperature of 2,750.degree. F. For powdered materials of high melting point, the criterion for "impact fusion" is met. But, for a metal such as aluminum with a melting point of about 1,200.degree. F., particle melting limits the accelerating nozzle length to less than that required to reach maximum particle velocity. However, my U.S. Pat. No. 5,120,582 teaches in certain examples that combustion pressure increases may be achieved in a simple manner using compressed air and fuel oil in place of propane such that, for a combustion pressure of 1,200 psig, the supersonic jet stream reach fully expended velocities in the range of Mach 4.5 (7,400 ft/sec). Such leads to particle impact velocities on substrates of over 4,000 ft/sec, and the coatings on the substrate improve in quality nearly directly proportional to impact velocity.